tcp_sack_permitted

Description

If set to 2, TCP always sends a SYN segment with the selective
acknowledgment (SACK) permitted option. If TCP receives a SYN segment with
a SACK-permitted option and this parameter is set to 1, TCP responds with
a SACK-permitted option. If the parameter is set to 0, TCP does not send a
SACK-permitted option, regardless of whether the incoming segment contains
the SACK permitted option.

Refer to RFC 2018 for information on the SACK option.

Default

2 (active enabled)

Range

0 (disabled), 1 (passive enabled), or 2 (active enabled)

Dynamic?

Yes

When to Change

SACK processing can improve TCP retransmission performance
so it should be actively enabled. Sometimes, the other side can be confused
with the SACK option actively enabled. If this confusion occurs, set the value
to 1 so that SACK processing is enabled only when incoming connections allow
SACK processing.